Failsafe bar connection

Abstract

A failsafe bar connection of a screw actuator includes a failsafe bar for transmitting load via a secondary load path, the failsafe bar having a bar-end with a convex thrust surface; and an attachment part for coupling load to an aircraft structure, the attachment part having a socket for retention of the bar-end, the socket providing a concave thrust surface. A contact mechanism is provided to monitor relative displacement of the convex thrust surface within the concave thrust surface in a direction along an axis of the attachment part. In this way, changes in backlash and loading of the secondary load path can be detected. The contact mechanism comprises a protuberance extending from the bar-end along the main rotational axis of the failsafe bar and a displaceable contact surface provided by a displaceable member in the attachment part, the relative position of which is monitored by a displacement sensor.

The invention claimed is: 
     
       1. A failsafe bar connection of a screw actuator comprising:
 a failsafe bar for transmitting load via a secondary load path, the failsafe bar having a bar-end with a convex thrust surface; and 
 an attachment part for coupling load to an aircraft structure, the attachment part having a socket for retention of the bar-end, the socket providing a concave thrust surface, 
 wherein a contact mechanism is provided to monitor relative displacement of the convex thrust surface within the concave thrust surface in a direction along an axis of the attachment part; 
 wherein the contact mechanism comprises a protuberance protruding from the bar-end which is in point contact with a displaceable contact surface of the contact mechanism. 
 
     
     
       2. A failsafe bar connection as claimed in  claim 1 , wherein the point contact lies on an axis of rotation of the failsafe bar. 
     
     
       3. A failsafe bar connection as claimed in  claim 1 , wherein the protuberance comprises a convex distal portion. 
     
     
       4. A failsafe bar connection as claimed in  claim 1 , wherein the protuberance is sized to extend across a gap present, at least initially, between the bar-end and the socket. 
     
     
       5. A failsafe bar connection as claimed in  claim 1 , wherein the displaceable contact surface is moveable in a direction along the axis of the attachment part, internally of the attachment part. 
     
     
       6. A failsafe bar connection as claimed in  claim 5 , wherein the displaceable contact surface is part of a plunger or a lever which is biased towards the protuberance. 
     
     
       7. A failsafe bar connection as claimed in  claim 1 , wherein the displaceable contact surface is arranged centrally within the attachment part. 
     
     
       8. A failsafe bar connection as claimed in  claim 1 , wherein the contact mechanism comprises a displacement sensor which measures the displacement of a target to monitor the relative movement of the displaceable contact surface and in turn the relative displacement of the convex thrust surface within the concave thrust surface. 
     
     
       9. A method of monitoring the operation of a failsafe bar connection in a screw actuator, the method comprising:
 arranging a bar-end of a failsafe bar within a socket of an attachment part, the bar-end providing a convex thrust surface for transmitting load along a secondary load path through engagement of a concave thrust surface of the attachment part, 
 characterised by monitoring the relative displacement of the convex thrust surface within the concave surface thrust surface in a direction along an axis of the attachment part using a contact mechanism; 
 wherein the monitoring comprises monitoring the movement of a displaceable contact surface in a direction along the axis of the attachment part, the movement being caused by a protuberance protruding from a bar-end of the failsafe bar along an axis of rotation of the failsafe bar. 
 
     
     
       10. A method as claimed in  claim 9 , wherein the protuberance is in point contact with the displaceable contact surface, the point contact preferably lying on an axis of rotation of the failsafe bar. 
     
     
       11. A method as claimed in  claim 10 , wherein the displaceable contact surface is provided on a plunger or a lever and is arranged for displacement along the axis of the attachment part and the method includes biasing the displaceable contact surface towards the protuberance. 
     
     
       12. A method as claimed in  claim 9 , wherein the monitoring comprises using a displacement sensor to detect relative displacement of the convex thrust surface through measuring displacement of a target linked to the displaceable contact surface. 
     
     
       13. A method as claimed in  claim 12 , wherein the displacement sensor and target are provided externally of the attachment part, and wherein relative displacement of the convex thrust surface is conveyed to the displacement sensor via a portion of a displaceable member which extends through a hole in a wall of the attachment part. 
     
     
       14. A method as claimed in  claim 9 , wherein the monitoring comprises continuous monitoring. 
     
     
       15. A failsafe bar connection as claimed in  claim 1 , wherein the protuberance comprises a ball-shape. 
     
     
       16. A failsafe bar connection as claimed in  claim 7 , wherein the displaceable contact surface is arranged to align with the concave thrust surface of the attachment part, at least initially. 
     
     
       17. A failsafe bar connection as claimed in  claim 8 , wherein the displacement sensor is mounted externally on the attachment part, and/or wherein the sensor comprises an LVDT sensor. 
     
     
       18. A method as claimed in  claim 10 , wherein the point contact lies on an axis of rotation of the failsafe bar. 
     
     
       19. A method as claimed in  claim 14 , wherein the continuous monitoring comprises outputting a signal to a computer, both during normal operation of a screw actuator and after loading via the secondary load path has begun.